Arylamine-based dyes for p-type dye-sensitized solar cells

Article abstract of DOI:10.1021/ol202014x

New arylamine-based sensitizers for p-type dye-sensitized solar cells (DSSCs) have been synthesized and used for p-type DSSCs. The best conversion efficiency reaches ～0.1%. Sensitizers with two anchoring carboxylic acids lead to higher open-circuit voltag

Full text of DOI:10.1021/ol202014x

ORGANIC
LETTERS
2011
Vol. 13, No. 18
4930–4933
Arylamine-Based Dyes for p-Type
Dye-Sensitized Solar Cells
Yung-Sheng Yen,† Wei-Ting Chen,†,‡ Chih-Yu Hsu,† Hsien-Hsin Chou,† Jiann T. Lin,*,†
and Ming-Chang P. Yeh*,‡
Institute of Chemistry, Academia Sinica, Nankang, Taipei 11529, Taiwan, and
Department of Chemistry, National Taiwan Normal University, Taipei, Taiwan 117
jtlin@chem.sinica.edu.tw; cheyeh@scc.ntnu.edu.tw
Received July 26, 2011
ABSTRACT
New arylamine-based sensitizers for p-type dye-sensitized solar cells (DSSCs) have been synthesized and used for p-type DSSCs. The best
conversion efficiency reaches ∼0.1%. Sensitizers with two anchoring carboxylic acids lead to higher open-circuit voltages, short-circuit currents,
and energy conversion efficiencies.
Dye-sensitized solar cells (DSSCs) have attracted con-
€
Sensitizers play an important role in DSSCs.5 One way
to improve light-harvesting of DSSCs is to use dyes
absorbing in different spectral regions.6 Similar to tandem
organic photovoltaic cells,7 tandem DSSCs with a p-type
semiconductor such as NiO8 as the photocathode also
allow the use of dyes having complementary spectral
coverage. A tandem DSSC with both electrodes photo-
active is predicted to have efficiency as high as 43%.9
Accordingly, there is increasing interest in p-type DSSCs.8c
1
siderable attention after Gratzel’s report because of the
following reasons: (1) efficient use of solar energy can meet
our increasing energy demand; (2) solar energy is envi-
ronmentally friendly; (3) organic sensitizers own advan-
tages of easy production and design versatility. Up to now,
DSSCs based on ruthenium dyes,2 Zn-porphyrin dye,3 and
metal-free dyes4 have promising conversion efficiencies,
reaching 11 and 10%.
(6) (a) Lee, K.-M.; Hsu, Y.-C.; Ikegami, M.; Miyasaka, T.; Justin
Thomas, K. R.; Lin, J. T.; Ho, K.-C. J. Power Sources 2011, 196, 2416.
(b) Fan, S.-Q.; Kim, C.; Fang, B.; Liao, K.-X.; Yang, G.-J.; Li, C.-J.;
Kim, J.-J.; Ko, J. J. Phys. Chem. C 2011, 115, 7747. (c) Yum, J.-H.;
† Academia Sinica.
‡ National Taiwan Normal University.
€
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Pootrakulchote, N.; Albabaei, L.; Ngoc-le, C.-h; Decoppet, J.-D.; Tsai,
€
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r
10.1021/ol202014x
Published on Web 08/22/2011
2011 American Chemical Society

The efficiencies of p-type DSSCs still remain low due to
slow hole mobility in popularly used NiO electrodes and
fast charge recombination and the non-ideal nature of the
redox mediator, iodide.5,8c For future applications, more
studies on p-type DSSCs are needed.
After formylation, the aldehyde was converted to the
desired product via condensation with DETB. Pinnick
oxidation16 was used for oxidation of aldehyde to form
carboxylic acid, and thiophene and formyl entities were
incorporated in two steps to avoid using a microwave
reactor. The compounds were isolated in 30ꢀ78% yields.
In p-type DSSCs, the electron pumped to the LUMO
(lowest unoccupied molecular orbital) of the dye will reduce
the oxidized redox mediator, and the hole of the HOMO
(highest occupied molecular orbital) will inject into the
valence band of NiO. Significant progress has been made
on p-type DSSCs recently.8b,10 Earlier, we developed some
amine-based metal-free sensitizers for n-type DSSCs.11 In
view of the strong donating character of the arylamine
moiety, we extended our studies to arylamine-based p-type
sensitizers. We are interested in p-type dyes with two
anchoring groups because they may not only bind the
NiO surface more firmly but also facilitate hole injection.
Compared to n-type congeners,12 p-type sensitizers with
multi-anchoring groups are still very rare.8b,13 Herein we
report the syntheses of new sensitizers and their applica-
tions in p-type DSSCs.
New dyes (Figure 1) synthesized are of two types: (1)
with one anchoring group and one acceptor (1, 2, and 3);
(2) with two anchoring groups and one acceptor (4, 5, and
6). The synthetic routes to the dyes are depicted in Scheme 1.
For 1ꢀ3, Pd-catalyzed aromatic CꢀN coupling14 between
ester-containing arylamine and formyl-containing aryl
bromide proceeded first, and subsequent acidification
provided carboxylic acid. Condensation of aromatic alde-
hyde with 1,3-diethyl-2-thioxodihydropyrimidine-4,6-
dione (DETB) or malononitrile gave the final product.
For 4 and 6, Pd-catalyzed Suzuki CꢀC coupling15 was
carried out on the arylamine-containing ester. Subsequent
formylation and condensation with malononitrile led to
the desired product. For compound 5, arylamine with two
ester groups was synthesized from primary amine and
bromobenzoate via Pd-catalyzed aromatic CꢀN coupling.
Figure 1. Structures of the dyes.
The absorption spectra and data are displayed in Figure 2
and Table 1, respectively. The absorption at >400 nm is
attributed to πꢀπ* transition mixed with charge transfer
transition from amine to dicyanovinyl unit. 1 and 4 have
the most blue-shifted absorption among all, possibly due
to the shorter spacer in the latter and the weaker acceptor
in the former. Prominent red shift of theabsorption spectra
upon replacing the dicyanovinyl entity with 4-methyli-
dene-3-phenylisoxazolone is consistent with the trend ob-
served in nonlinear optical chromophores.17 Slight blue
shift of the absorption in5 thanin 3 can be attributedto the
weakeneddonating powerofthe arylamine (videinfra) due
to the presence of an extra carboxylic acid in the former.
Compared with 1, it is evident that elongation of the
conjugation chain by adding extra thiophene (2 and 6) or
extra acceptor (S) results in better charge delocalization.
The charge transfer character in these compounds is con-
firmed by the density functional calculations (Table S1 and
Figure S1 in the Supporting Information).
The lowest-lying electronic transitions, S1 (mainly
HOMO f LUMO), have significant oscillator strength
(f) and extent of charge separation (i.e., changes in Mulli-
ken charge in the transition) from the arylamine to the
acceptor. There is very prominent population of the
HOMO at the arylamine side extending to the carboxylic
acid entity, and population of the LUMOs at the dicya-
novinyl side is also evident. It is interesting to note that
introduction of a cyclopentadithiophene moiety (CDT) in
the spacer (6) or an extra acceptor (S) increases the
absorption intensity dramatically.
(10) (a) Qin, P.; Zhu, H.; Edvinsson, T.; Boschloo, G.; Hagfeldt, A.;
Sun, L. J. Am. Chem. Soc. 2008, 130, 8570. (b) Qin, P.; Linder, M.;
Brinck, T.; Boschloo, G.; Hagfeldt, A.; Sun, L. Adv. Mater. 2009, 21,
2993. (c) Li, L.; Gibson, E. A.; Qin, P.; Boschloo, G.; Gorlov, M.;
Hagfeldt, A.; Sun, L. Adv. Mater. 2010, 22, 1759.
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K.-C.; Lai, C.-H.; Cheng, Y.-M.; Chou, P.-T. Chem. Mater. 2008, 20,
1830. (b) Lin, J. T.; Chen, P.-C.; Yen, Y.-S.; Hsu, Y.-C.; Chou, H.-H.;
Yeh, M.-C. P. Org. Lett. 2008, 11, 97. (c) Chen, C.-H.; Hsu, Y.-C.; Chou,
H.-H.; Justin Thomas, K. R.; Lin, J. T.; Hsu, C.-P. Chem.;Eur. J. 2010,
16, 3184.
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Mosconi, E.; Yum, J.-H.; Xianxi, Z.; Nazeeruddin, M. K.; Gratzel, M.
Energy Environ. Sci. 2009, 2, 1094. (b) Heredia, D.; Natera, J.; Gervaldo,
M.; Otero, L.; Fungo, F.; Lin, C.-Y.; Wong, K.-T. Org. Lett. 2010, 11,
12. (c) Sahu, D.; Padhy, H.; Patra, D.; Yin, J.-F.; Hsu, Y.-C.; Lin, J. T.;
Lu, K.-L.; Wei, K.-H.; Lin, H.-C. Tetrahedron 2011, 67, 303.
(13) A paper on sensitizers with two anchoring groups appeared
while this paper was in preparation: Ji, Z.; Natu, G.; Huang, Z.; Wu, Y.
Energy Environ. Sci. 2011, 4, 2818.
(14) (a) Yamamoto, T.; Nishiyama, M.; Koie, Y. Tetrahedron Lett.
1998, 39, 2367. (b) Hartwig, J. F.; Kawatsura, M.; Hauck, S. I.;
Shaughnessy, L. M.; Alcazar-Roman, J. J. Org. Chem. 1999, 64, 5575.
(15) (a) Miyaua, N.; Suzuki, A. Chem. Commun. 1979, 866. (b)
Suzuki, A. J. Organomet. Chem. 1999, 576, 147.
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1981, 37, 2091. (b) Nakamura, S.; Sato, H.; Hirata, Y.; Watanabe, N.;
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Mater. 1997, 9, 132.
Org. Lett., Vol. 13, No. 18, 2011
4931

Scheme 1. Synthesis of the Dyes
Table 1. Electrooptical Parameters of the Dyes
λabs
Eox
(ΔEp),a,b
mV
(ε ꢁ 10ꢀ4 Mꢀ1 cmꢀ1 a
)
HOMOc, LUMOd,
dye
nm
V
V
1
2
338 (2.46), 461 (3.51) 594 (120)
1.29
1.27
ꢀ0.96
ꢀ0.89
340 (3.80), 369 (4.15), 565 (128), 809
489 (5.88)
(139)
3
329 (3.59), 364 (3.18), 610 (146)
508 (6.08)
1.31
ꢀ0.77
4
5
6
348 (3.59), 460 (3.45) 790 (103)
356 (4.70), 504 (3.72) 674 (111)
350 (3.15), 514 (6.76) 553 (118)
715 (118)
1.49
1.37
1.25
ꢀ0.86
ꢀ0.68
ꢀ0.91
S
347 (3.67), 484 (6.75) 718 (137)
1.42
ꢀ0.81
Figure 2. Absorption spectra of the dyes in tetrahydrofuran.
a Recorded in THF solutions at 298 K. b Eox = 1/2(Epa þ Epc), ΔEp
= Epa ꢀ Epc, where Epa and Epc are anodic and cathodic potentials,
respectively. Oxidation potential is referenced to that of ferrocene (E1/
2(ox) = 293 mV vs Ag/AgNO3), which was used as an internal reference.
Scan rate: 100 mV/s. c HOMO level vs NHE. d LUMO level vs NHE.
The electrochemical data and the cyclic voltammograms
of the dyes are displayed in Table 1 and Figure S2,
respectively. Except for 2 and 6, all compounds exhibit a
quasi-reversible one-electron redox wave attributable to
the oxidation of the arylamine. 2 (or 6) exhibits two quasi-
reversible one-electron redox waves due to the oxidation of
arylamine and bithiophene (or CDT), respectively. The
first oxidation potentials (Eox) of the compounds decrease
in the order of 4 > 3 > 1 > 5 > 2 > 6. The presence of
electron excessive bithiophene (or CDT) significantly low-
ers the oxidation potential of 2 (or 6). In comparison, the
presenceoftwo carboxylic acids leads toa higher oxidation
potential (3, 610 mV; 5, 670 mV vs ferrocene) due to
depletion of the electron density of arylamine. It is also
evident that the stronger electron-withdrawing DETB
more effectively raises the oxidation potential of arylamine
than dicyanovinyl unit (1vs 3). TheHOMO(1.25toꢀ1.49 V
vs NHE) and LUMO (ꢀ0.68 to ꢀ0.96 V vs NHE) energy
levels of the compounds were estimated from the oxidation
potential and the absorption band edge (Table 1). The
former is below that of the top of the valence band of NiO
(0.54 V vs NHE),8c and the latter is above that of the redox
mediator (0.40 V vs NHE). Therefore, hole injection and
dye regeneration should occur readily.
The p-type DSSCs with an effective area of 0.25 cm2 of
nanostructured NiO film were fabricated from these dyes.
The electrolyte was composed of 0.1 M I2/1.0 M LiI/0.5 M
tert-butylpyridine in CH3CN. The device performance
statistics under AM 1.5 illumination are listed in Table 2.
The cell using a known sensitizer (S,10a Figure 1) was also
fabricated for comparison. Figure S3 shows the photo-
currentꢀvoltage (JꢀV) curves of the cells. Figure 3 com-
pares the incident photon-to-current conversion effi-
ciencies (IPCE) of the dyes on NiO. The conversion
4932
Org. Lett., Vol. 13, No. 18, 2011

efficiencies of the cells reach 50ꢀ92% of the cell based on
S, and the highest IPCE value is achieved by 4 (maximum
of ca. 28% at 482 nm). The spectra of the dyes adsorbed on
the NiO surface are shown in Figure S4. The dye adsorbing
amount and the hole injection ability to the NiO result in
different photocurrent output as well as light-harvesting
ability. The hole injection ability can be realized through
the EIS analysis of the DSSCs under illumination (Figure
S5a). From the intermediate frequency semicircle (1ꢀ102
Hz), the interfacial charge transfer resistance at the NiO/
dye/electrolyte interface (Rct2) can be calculated.
surface but lower photocurrent compared to others, pos-
sibly due to less efficient hole injection or light harvesting.
Despite blue-shifted absorption spectra, 4 has high photo-
current output, which is consistent with its low Rct2 value.
The low Rct2 values of 5 and 6 are also attributed to the
presence of two anchoring groups.
The dyes with two anchoring groups (4 to 6) that have
significantly higher open-circuit voltages deserve atten-
tion. The low efficiency of DSSC based on C343 was
attributed to a fast charge recombination of the reduced
dye with the hole in the valence band of NiO instead of
charge recombination between the electrolyte and the hole
(dark current).18 We believe that suppression of dark
current may be also taken into consideration for the cell
performance. More efficient suppression of dark current of
N719 than Z907 was suggested to be due to more effective
coverage of TiO2 for prevent electrolytes from approach-
ing the TiO2 surface.19 Similarly, the two anchoring groups
in 4ꢀ6 may effectively cover the naked NiO surface from
being exposed to the electrolyte.
Our argument is supported by the EIS measured in the
dark (Figure S5b). The intermediate frequency semicircle
represents the charge transfer phenomena between the NiO
surface and the electrolyte (i.e., dark current). Efficient
suppression of the dark current leads to a larger semicircle.
Variation of the NiO Fermi level caused by the surface dipole
cannot be excluded, however.20 Two carboxylic groups
facilitate hole injection, as evidenced from the HOMO of
the molecule, which has an electron population extending to
the carboxylic groups (vide supra). Besides suppression of
dark current, sensitizers with two anchoring groups may also
enhance dye bonding on the surface of the photoelectrode.
In conclusion, new arylamine-based sensitizers for
p-type DSSCs have been synthesized. The p-type DSSCs
using these sensitizers and NiO as the photocathode have
efficiencies ranging from 0.053 to 0.093%. The sensitizers
with two anchoring carboxylic acids have higher VOC and
Table 2. DSSCs Performance Parameters of the Dyes
dye loading,
VOC
,
JSC, mA/
cm2
10ꢀ7 mol/
cm2
Rct2,
dye
V
FF
η, %
Ω
1
2
3
4
5
6
S
0.105
0.115
0.113
0.125
0.122
0.131
0.132
1.59
1.39
1.38
2.25
2.18
2.05
2.31
0.359 0.060
0.363 0.058
0.340 0.053
0.331 0.093
0.346 0.092
0.324 0.087
0.331 0.101
1.53
1.58
1.54
1.30
1.19
1.08
1.29
183
160
223
124
134
135
146
JSC values as well as better efficiencies.
Acknowledgment. We acknowledge the support of the
Academia Sinica (AC) and NSC (Taiwan), and the Instru-
mental Center of Institute of Chemistry (AC).
Figure 3. IPCE plots for the DSSCs using PT and S dyes.
Supporting Information Available. Synthetic proce-
dures and characterization for new compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
The Rct2 values obtained by employing the equivalent
circuit for the curves fitted the impedance spectra of the
DSSCs (Figure S5a) are listed in Table 2 along with the dye
loading amount. 1ꢀ3 have higher dye density on NiO
(19) Mori, S. N.; kubo, W.; Kanzaki, T.; Masaki, N.; Wada, Y.;
Yanagida, S. J. Phys. Chem. C 2007, 111, 3522.
(20) (a) Heimel, G.; Romaner, L.; Zojer, E.; Bredas, J.-L. Acc. Chem.
Res. 2008, 41, 721. (b) Braun, S.; Salaneck, W. R.; Fahlman, M. Adv.
Mater. 2009, 21, 1450.
€
(18) Moranderia, A.; Boschloo, G.; Hagfeldt, A.; Hammarstrom, L.
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